But the Bene Tleilaxu Face Dancers are a neat part of the Dune world, and I think they're one of the easier things to adapt to Swords & Wizardry:
Use the Doppleganger
With our philosophy of only add / change / remove material when absolutely necessary… I just don't see much reason not to use the Doppleganger for a Facedancer.
I'd perhaps give them a normal human movement rate of 12" (instead of 9") and lose their natural 1d12 claw attack and high AC… but otherwise isn't that what a Facedancer is: A Doppleganger?
Once again, it would be easy to add a "new" monster and create a stat block for it and add to the overall bulk of the game... but is that really necessary? Then again, I'd boil down the S&W to something like this anyway:
Hit Dice: 4
Saving Throw: 13 (5 against any magic)
Special: Mimics shape, immune to sleep and charm
Anything that's not listed is assumed to be "human standard" (AC 9, Movement 12", etc). There's no reason to list most monsters as having a movement of 12"... just list that info for the ones that don't.